Generally, in a furnace which has high dust concentration and where a continuous treatment is performed like a rotary kiln provided in a cement production equipment, a radiation thermometer or the like which can measure a temperature in a contactless manner is used for temperature measurement of an object to be heated. This radiation thermometer is widely used in a baking furnace other than the rotary kiln or high-temperature processes.
However, the radiation thermometer has a problem that presence of dust between the object to be heated which is a measurement target and an observer leads to an influence of attenuation of radiation light caused by the dust and the radiation light from the dust itself, and a temperature of the object to be heated cannot be accurately measured. A two-color thermometer known as a non-contact thermometer obtains a temperature from a radiance ratio at two wavelengths, and this two-color thermometer can ignore an influence of attenuation of the radiation light caused by the dust which does not change the radiance ratio, but it cannot ignore an influence of the radiation light from the dust which changes the radiance ratio.
The above-described problem according to the temperature measurement in the furnace having high dust concentration can likewise occur in other baking furnaces than the cement firing furnace as a matter of course. To eliminate such a problem, for example, a temperature measuring method which enables assuredly measuring a liquid level temperature of molten slag in a furnace having high soot concentration is disclosed (see, e.g., Patent Literature 1). According to this temperature measuring method, of radiant light radiated from a liquid level of molten slag accommodated in a furnace, radiant light in an intermediate-infrared region or a far-infrared region is concentrated on a photoelectric element, an output voltage having an amplitude corresponding to intensity of the incoming radiant light is generated from the photoelectric element, and the liquid level temperature of the molten slag is determined on the basis of this output voltage value and Planck's law of radiation. Further, according to this temperature measuring method, radiant lights having two or more different wavelengths are used.
Furthermore, there is disclosed a flame light emission measuring apparatus using a condensing optical system in local measurement which is generally adopted to measure a fine structure of a flame (see, e.g., Patent Literature 2). This measuring apparatus is characterized by having a condensing optical system constituted of a single optical system which condenses light emission from a plurality of measuring points of a flame onto corresponding condensing points on a condensing surface and a light emission measuring system in which the light emission from the plurality of measuring points condensed on the condensing points respectively is measured.